Initial investigation of bimetal hydroxysulfide as a new anode material for efficient sodium-ion storage

Various transition metal compounds (TMCs) such as metal oxide, metal sulfide, and metal selenide have been developed owing to the continous efforts devoted for exploring novel and efficient anode materials for alkali metal-ion (Li⁺/Na⁺) batteries. In this study, to synthesize a new anode candidate, multicomponent hybridization of metal hydroxide and metal chalcogenide is applied for inducing the active reaction with Na-ions by formation of heterostructured metal hydroxide/metal chalcogenide during the first cycle. As a first target material, cobalt-iron hydroxysulfide was prepared through room-temperature sulfurization by simply immersing cobalt-iron hydroxide into a solution of Na₂S. To understand the conversion reaction mechanism of cobalt-iron hydroxysulfide in sodium-ion storage in detail, various in-situ electrochemical analysis and ex-situ TEM and XPS analysis were conducted. The reversible conversion reaction after the first cycle can be estimated by the following equation: Co + Fe + 4NaOH + 4Na₂S ↔ Co(OH)₂ + Fe(OH)₂ + CoS₂ + Na_xFeS₂ + (12-x)Na⁺ + (12-x)e⁻. Heterostructured metal hydroxide/metal sulfide nanocomposite was formed after the initial cycle, which provided excellent electrochemical properties. Moreover, to efficiently apply cobalt-iron hydroxysulfide, hollow carbon nanospheres uniformly embedding cobalt-iron hydroxysulfide were synthesized by a vacuum process. Yolk-shell structured cobalt-iron hydroxysulfide-C composite nanospheres showed a stable cycle performance (285 mA h g⁻¹ after 200 cycles at a current density of 1.0 A g⁻¹) and superior rate capability (307 mA h g⁻¹ at a high current density of 5.0 A g⁻¹).